A standard for a wireless local area network (WLAN) technology has been developed as IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard. IEEE 802.11a or IEEE 802.11b uses an unlicensed band in 2.4 GHz or 5 GHz, IEEE 802.11b provides transmission speed of 11 Mbps and IEEE 802.11a provides transmission speed of 54 Mbps. IEEE 802.11g provides transmission speed of 54 Mbps in a manner of applying Orthogonal Frequency Division Multiplexing (OFDM) in 2.4 GHz. IEEE 802.11n provides transmission speed of 300 Mbps for 4 spatial streams in a manner of applying Multiple Input Multiple Output-OFDM (MIMO-OFDM). IEEE802.11n supports a channel bandwidth up to 40 MHz. In this case, IEEE802.11n provides transmission speed of 600 Mbps.
Meanwhile, a TV white space (TV WS) is a frequency assigned to a broadcast TV and includes an Ultra High Frequency (UHF) band and a Very High Frequency (VHF). The TV WS means a frequency band permitted to an unlicensed device to use the frequency under a condition that the unlicensed device does not impede a communication of a licensed device operating in the corresponding frequency band. In particular, a device operating as the unlicensed device can use an available channel or an available frequency band not used by the licensed device in the corresponding frequency band. In this case, the licensed device may include a TV, a wireless microphone, and the like. The licensed device may be called a licensed user, an incumbent user, or a primary user and the like.
The unlicensed device wishing to use the TV WS should provide a protection function for the licensed device. Hence, the unlicensed device should check whether the licensed device occupies a corresponding band before starting a signal transmission in the TV WS to obtain available channel information.
To this end, the unlicensed device can check whether the corresponding band is used by the licensed device in a manner of performing a spectrum sensing. A spectrum sensing mechanism includes an energy detection scheme, a feature detection scheme, and the like. If strength of a signal received from a specific channel is greater than a prescribed value or a DTV (digital TV) preamble is detected, the unlicensed device can judge that the specific channel is currently used by the licensed device. And, if it is judged that the licensed device currently uses a channel adjacent to the channel currently using, the unlicensed device should lower a transmit power depending on an amount of interference emitted to a neighboring band in some cases.
If the unlicensed device depends on a frequency sensing only to obtain available channel information in the TV WS, the burden resulted from sensing for an operation in the TV WS laid on the unlicensed device increases and a procedure may be delayed. Hence, the unlicensed device may access a geo-location database via the internet or a dedicated network and may be then able to obtain channel list information available in a corresponding area. The geo-location database is a database configured to store and manage information on registered licensed devices, geographical location of the licensed devices, and channel usage information that changes dynamically according to time of a channel use.